The present invention relates to a semiconductor light-emitting element making use of a compound semiconductor having a Wurtzite-type crystal structure and also to a semiconductor light-emitting device, an image display device and an illumination device, each using such a semiconductor light-emitting element making use of the compound semiconductor as mentioned above. More particularly, the present invention relates to a semiconductor light-emitting element capable of emitting multi-color or white light by use of a compound semiconductor having a Wurtzite-type crystal structure and also to a semiconductor light-emitting device, an image display device and an illumination device.
For a semiconductor light-emitting element, there is known an element which includes, entirely on a sapphire substrate, a builtup Iayer including a low temperature buffer layer, an n-type contact layer made of GaN doped with Si (silicon), an n-type clad layer made of Si-doped GaN formed on the contact layer, an active layer made of Si-doped InGaN, a p-type clad layer made of Mg-doped AlGaN, a p-type contact layer made of Mg (magnesium) doped GaN and the like. Commercially available products having such a structure as mentioned above have been mass-produced including blue and green LEDs (Light Emitting Diodes) with wavelengths ranging from 450 nm to 530 nm.
By the way, an image display device can be constituted by providing individual pixels through combinations of blue, green and red diodes and lasers, arranging the pixels in matrices and independently driving them. Moreover, blue, green and red light-emitting elements are subjected to simultaneous light emission and, thus, can be utilized as a white light-emitting device or an illumination device. In particular, a light-emitting element using a nitride semiconductor has a band gap energy ranging from about 1.9 eV to about 6.2 eV. Thus, it becomes possible to provide a full-color display by use of only one material, so that studies on a multi-color light-emitting element have been in progress.
For a technique of forming multi-color light-emitting elements on the same substrate, there is known an element wherein a number of active layers having different band gap energies depending on the difference in light-emitting wavelength are built up, and while a common electrode is used at the substrate side, counter electrodes are formed individually for different colors. Moreover, there is also known an element which has such a structure that a substrate has a stepped surface for electrode terminals wherein the respective steps corresponds to different colors. The element wherein a number of pn-junctions are built up in such a way as set out above has the possibility that a light-emitting element works as a thyristor within the same element. In order to prevent the thyristorlized operations, there is known an element wherein a groove is formed at every stepped portion for isolating the respective colors from one another. Such an element is disclosed, for example, in Japanese Patent Laid-open No. Hei 9-162444. The light-emitting element disclosed in Japanese Patent Laid-open No. Hei 9-92881 enables multi-color light emission wherein an InGaN layer is formed on an alumina substrate via an AlN buffer layer wherein Al is doped at a part of the InGaN layer thereby causing blue light to be emitted, P is doped at another part to cause red light to be emitted and a non-doped region of the InGaN layer is provided as a green light emission region, thus permitting multi-color light emission.
However, with such a semiconductor light-emitting element having such a structure as set out above, the fabrication process becomes complicated, so that the light-emitting element cannot be formed in high precision. Moreover, because crystallinity is degraded, good light-emitting characteristics cannot be obtained.
More particularly, with an element of the type wherein a groove is formed in every stepped portion for an intended color to permit isolation of individual colors, the regions of the respective active layers whose band gap energies differ from each other are isolated through etching, thus making it necessary to perform multiple anisotropic etching cycles. In general, however, a substrate and a semiconductor layer may suffer degradation in crystallinity depending on the manner of dry etching and, especially, where multiple etching cycles are performed, an element is formed through crystal growth from the surface of a substrate, where exposed by etching, in a second and subsequent etching cycles, thus making it difficult to keep crystallinity at a high level. When multiple etching cycles are carried out, the numbers of the steps of mask alignment and etching increase correspondingly, resulting in the increased fabrication costs of the element.
Further, in a technique of selectively doping an impurity in a single active layer formed on a substrate where openings of a mask layer are utilized, for example, for the selective doping, the layout of the respective emission colors has to be determined while taking into account a positional error such as an opening-forming margin of the mask layer. Where the error is taken into account beforehand, it is necessary to take a sufficient distance between the regions of different emission colors. Accordingly, where it is intended to form a minute or fine element, the regions for emission of different colors increase in size. In addition, it is as a matter of fact that the steps increase in number owing to the selective doping.
Hence, the present invention has been made in view of the problems set out above and provides a semiconductor light-emitting element having such a structure that is formed in high precision and does not invite any degradation of crystallinity without complicating a fabrication process thereof. The present invention is also directed to a semiconductor light-emitting device, along with the provision of an image display device and an illumination device each having a structure using such excellent semiconductor light-emitting element and semiconductor light-emitting device as mentioned above.